In recent years, demand for a memory having characteristics of nonvolatility, a large storage capacity, a low voltage operation and low power consumption is increasing as a result of rapid spread of portable telephone or the like. The MRAM (magnetic random access memory) is anticipated as a memory having these characteristics. A storage element of the MRAM includes a TMR (tunnel magnetoresistance) element. Each TMR element has, for example, a configuration as shown in FIG. 16. The TMR element is formed by successively stacking a fixed ferromagnetic layer (pin layer) 901, a tunnel insulation layer 902, and a free ferromagnetic layer (free layer) 903. The direction of magnetization of the pin layer 901 is fixed at the time of manufacture. On the other hand, the magnetization direction of the free layer 903 can be reversed by magnetic fields generated by currents in wires. For example, the magnetization direction of the free layer 903 can be reversed by magnetic fields generated by a current flowing through a bit line BL and a current flowing through a word line WL. “1” or “0” is assigned according to the direction of the magnetization. When the relative magnetization direction between the pin layer 901 and the free layer 903 is parallel (“0” in FIG. 16), the electrical resistance is low. When the relative magnetization direction between the pin layer 901 and the free layer 903 is antiparallel (“1” in FIG. 16), the electrical resistance is high. By detecting the difference in electrical resistance, therefore, the state of the storage element can be read out.
A semiconductor memory device using the TMR element having such a configuration as a memory cell has a configuration in which a plurality of memory cells 904 are arranged in a matrix form as shown in FIG. 17A. The semiconductor memory device includes as components a plurality of bit lines BL 905 which extend in the lateral direction over the memory cells, and a plurality of word lines WL 906 which extend in the longitudinal direction under the memory cells. Each memory cell 904 includes the TMR element. When a current flows through each of a bit line BL and a word line WL disposed over and under a selected cell and a combination of magnetic fields HY and HX generated by respective currents satisfies a predetermined condition, the magnetization direction of the free layer can be reversed. A combination of the weakest magnetic fields required for magnetization reversal forms a curve called an asteroid curve as shown in FIG. 17B. (In FIG. 17B, reversal from “0” to “1” is assumed.) If a magnetic field of outside of the asteroid curve (i.e., a magnetic field in the “Reversal” region and “Multiple Write” region) is applied, writing into a selected cell S is conducted. For example, if an X direction magnetic field HDX and a Y direction magnetic field HDY shown in FIG. 17B is applied, magnetization reversal is caused because a magnetic field vector (HX, HY)=(HDX, HDY) in the selected cell S is in the reversal region. In other words, data “0” or “1” can be written by reversing the magnetization direction. In unselected memory cells UX and UY on the selected bit line and the selected word line, respectively, magnetization reversal is not caused at this time, because only the magnetic field HDX and HDY, respectively, which fall inside the asteroid curve (“Retention” region) is present. In other words, selective writing is conducted.
The magnetic field (HX, HY) can be rewritten with respect to a word line current IDY and a bit line current IDX by using the Ampere's law (I=H/2, r, where r is a distance between the center of wire and the center of the magnetic substance). A result of rewriting is shown in FIG. 17C. If the word line current IDY and the bit line current IDX are flowed, magnetization reversal is caused, because the current combination (IBL, IWL)=(IDX, IDY) in the selected cell S is in the reversal region. In other words, data “0” or “1” can be written by reversing the magnetization direction. In unselected memory cells UX and UY on the selected bit line and the selected word line, respectively, magnetization reversal is not caused at this time, because only the current IDX and IDY, respectively, which fall inside the asteroid curve (“Retention” region) flows. In other words, selective writing is conducted.
In the case of the MRAM, however, a large number of unselected cells are connected to the selected bit line BL 905 and the selected word line WL 906 as shown in FIG. 17A. If currents flow through the lines, therefore, these unselected cells are subject to the disturbing magnetic field. For example, if a write current in a checkered region (“Multiple Write” region) in FIG. 17C is flowed, writing into the unselected memory cells UX and UY is also conducted, because the current IBL in the unselected memory cell UX and the current IWL in the unselected memory cell UY go outside the asteroid curve. In other words, false writing is conducted. For conducting selective writing, therefore, it is necessary to let currents in the “Reversal” region represented by an unshaded portion in FIG. 17C flow, and accurate adjustment of write currents is necessary.
As prior technique reference relating to the present invention, the following can be mentioned.
JP-A-2001-195878
JP-A-2001-325791
JP-A-2002-008367
JP-A-2002-074974
JP-A-2002-170374
JP-A-2002-170375
JP-A-2002-170376
JP-A-2002-197852
JP-A-2003-257175